1. Field of the Invention
This invention relates to a diagnostic circuit of a data processing system and more specifically to an error detection system of a diagnostic circuit utilizing a scan-out function.
2. Description of the Prior Art
There is a scan-out function for outputting and displaying the internal condition of the circuits of various devices of a computer system. In order to use the scan-out function, an address is transferred to a flip-flop, gate or the like in a scan circuit in the target device. The address has a number of bits sufficient to select all the circuits in the devices, such as flip-flops and gates etc. The state of the desired circuits can be output by using the address to produce a combination of selection signals. Conditions of all of the circuits can be output by sequentially updating. The circuit condition is displayed or recorded if an error is generated, or used as instruction control information in an alternative use of the condition.
FIG. 1 outlines an example of structure of a computer system providing the scan-out function. Block 1 is a service processor (SVP), 2 is a system console interface unit (SCI), 3 is a central processing unit (CPU), 4 is a channel processor (CHP), 5 is a memory control unit (MCU) and 6 is a main memory unit (MSU).
The SCI 2 is provided with a scan-out control circuit and the CPU3, CHP4, MCU5 and MSU6 are provided with a scan-out circuit. The SCI 2 itself is also provided with a scan-out circuit. Each unit receives a plurality of selection signals selecting its scan-out circuits and sends to SCI 2 scan-out data indicating the condition of the scan-out circuit having the address designated a combination of the selection signals. The SVP 1 generates address of the circuit to be scanned, then sends it to SCI 2 in order to initiate execution of the scan-out control by SCI 2. Simultaneously, SCI 2 receives data output from the scan-out circuit which it passes on to be used in accordance with application purposes.
The SCI 2 is provided between SVP1 and CPU3, CHP4, MCU5 and MSU6 and has the function of controlling the transmission of the scan-out address and relaying output data from the scan-out circuit to the SVP 1. The SVP 1 is used for displaying the output of the scan-out circuit on a display unit, printing it with a printer, recording it in a file or for alternative control of instructions.
In a recent computer system, data output by the scan-out circuit is important information used for identifying the defective area of a failure occurs in the computer system. If the data is erroneous, one problem is that adequate or proper maintenance becomes impossible. Also, since such output can be used as control information by an instruction, if there is an error in the data, the result of the execution of the instruction is unpredictable and may cause serious problems.